We use block links throughout our website for advertisements, offers, etc.
I've recently learned that block links are officially only supported
starting with HTML5. So while the web browser handles them fine
(seemingly), and only irritate screen reader users (unfortunately), this
technically fails validation. How does this relate to the "R" in WCAG's
POUR principles (Robust)?

I see mention that under parsing, web pages should validate, with the HTML
conforming to specifications. Block links under a XHTML doctype would fail
this. Does this mean these pages fail compliance (especially if mandated)?
Additionally, if a page has validation errors, would this be an
accessibility compliance concern?